Hydrocarbons: Class 10 Chemistry

Key points

Hydrocarbons are compounds made of carbon and hydrogen only. They are the parent compounds of organic chemistry. The main sources are petroleum, natural gas and coal. Hydrocarbons are saturated (alkanes) or unsaturated (alkenes and alkynes), and some are aromatic (benzene).

General formulas and simple members:

• Alkanes: CₙH₂ₙ₊₂ with single bonds only, such as methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀.
• Alkenes: CₙH₂ₙ with one C=C double bond, such as ethene C₂H₄ and propene C₃H₆.
• Alkynes: CₙH₂ₙ₋₂ with one C≡C triple bond, such as ethyne C₂H₂ and propyne C₃H₄.

Alkanes are also called paraffins. The carbon atoms are sp³ hybridized, with four sigma bonds arranged in a tetrahedral shape, bond angle 109.5°. In alkenes the carbon atoms are sp² hybridized, planar, with bond angle about 120°. In alkynes they are sp hybridized and linear, with bond angle 180°. The C=C bond is shorter and stronger than a C−C bond.

Naming uses the longest chain with the double or triple bond. The suffix is −ane, −ene or −yne. The chain is numbered from the end that gives the multiple bond the lowest number. CH₃CH=CH₂ is propene, CH₃CH₂CH=CH₂ is but-1-ene, and CH≡CH is ethyne. An alkyl group is formed by removing one H from an alkane, for example methyl (−CH₃) and ethyl (−C₂H₅).

Physical properties: the first four alkanes are gases, C₅ to C₁₇ are liquids and higher members are solids. The boiling point rises with chain length because the weak van der Waals forces get stronger. A branched isomer boils lower than its straight chain isomer. All hydrocarbons are non-polar, insoluble in water, less dense than water and soluble in organic solvents.

Alkanes are fairly unreactive because C−C and C−H bonds are strong and non-polar. Their main reactions are combustion and substitution. Complete combustion gives carbon dioxide and water with heat: CH₄ + 2O₂ → CO₂ + 2H₂O. In limited oxygen, carbon monoxide or carbon (soot) forms, which is dangerous.

Substitution in alkanes: in sunlight or UV light, chlorine replaces hydrogen one atom at a time by a free radical mechanism. CH₄ + Cl₂ → CH₃Cl + HCl, and further to CH₂Cl₂, CHCl₃ (chloroform) and CCl₄. The steps are initiation (Cl₂ splits into two Cl radicals), propagation, and termination when radicals combine.

Preparation of alkanes: methane can be made in the laboratory by heating sodium ethanoate with soda lime (NaOH and CaO): CH₃COONa + NaOH → CH₄ + Na₂CO₃. Alkanes are also made by hydrogenation of alkenes using a nickel catalyst, and by cracking of larger alkanes.

Alkenes are made by dehydration of alcohols: C₂H₅OH heated with concentrated H₂SO₄ at about 170 °C gives ethene and water. Another route is dehydrohalogenation of an alkyl halide with alcoholic KOH. In industry ethene comes from cracking of petroleum fractions.

Alkenes and alkynes are more reactive because of the pi bond. Their typical reaction is addition:

• Hydrogenation: C₂H₄ + H₂ → C₂H₆ (Ni catalyst, about 200 °C). This is used to turn vegetable oils into ghee.
• Halogenation: C₂H₄ + Br₂ → CH₂Br−CH₂Br.
• Hydrohalogenation: C₂H₄ + HBr → C₂H₅Br.
• Hydration: C₂H₄ + H₂O → C₂H₅OH (steam, phosphoric acid catalyst).
• Alkynes add twice as much reagent, for example C₂H₂ + 2H₂ → C₂H₆.

Test for unsaturation: shake the compound with bromine water (orange) or with dilute alkaline KMnO₄ (purple, Baeyer's test). Alkenes and alkynes decolourize them quickly, while alkanes do not. Ethene and ethyne burn with a sooty yellow flame because of their higher percentage of carbon, whereas methane burns with a blue flame.

Ethyne is made in the laboratory by adding water to calcium carbide: CaC₂ + 2H₂O → C₂H₂ + Ca(OH)₂. It burns with a very hot flame in oxyacetylene torches used for welding and cutting metals. Ethene is used to make polythene and to ripen fruit. Polymerization joins many ethene molecules: nC₂H₄ → (−CH₂−CH₂−)ₙ.

Benzene, C₆H₆, is the simplest aromatic hydrocarbon. It is a planar ring of six carbon atoms with delocalized pi electrons, and all C−C bonds have the same length. It prefers substitution to addition, as in nitration with a mix of conc. HNO₃ and H₂SO₄ to give nitrobenzene. Benzene vapour is toxic and can cause cancer.

Study tip: Make a small table of alkane, alkene and alkyne with formula, bond, hybridization, shape and typical reaction, and revise it. Practise balancing combustion equations and mole calculations at STP.

Definitions and flashcards

Say the meaning first, then open the card to check.

Formula sheet

Print this page

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Using the wrong general formula, for example CₙH₂ₙ for alkanes.
• Writing substitution of alkanes as addition. Alkanes give substitution and alkenes give addition.
• Forgetting the conditions: UV light for chlorination, Ni for hydrogenation, 170 °C and conc. H₂SO₄ for dehydration of ethanol.
• Numbering the chain from the wrong end so that the double bond does not get the lowest number.
• Not balancing combustion equations, which loses marks in numericals.

Practise this chapter

Practice questions and timed test Printable worksheet